Fig. 2: Crystal orientation dependence of the 7th harmonic generation yield.

a Schematic of the monolayer WS₂ crystal structure and laser polarization axis (red arrow). The x(y)-axis corresponds to the zigzag (armchair) directions, respectively. The angle θ represents the counterclockwise rotation of the excitation laser polarization relative to the zigzag axis. b–f Crystal orientation dependence of seventh harmonics at increasing laser peak intensities: b 125 GW/cm², c 235 GW/cm², d 305 GW/cm², e 350 GW/cm², and f 380 GW/cm². For low driving power the harmonic response is isotropic and perturbative. At higher intensity in the transition to non-perturbative HHG, a distinct six-fold pattern emerges with emission along the armchair direction. At yet higher intensity the six-fold pattern is slightly less pronounced and rotated by 30°, exhibiting stronger harmonic intensity along the zigzag direction. The harmonic signals in (b), (c), (d), and (e) are magnified by factors of 200, 10, 2, and 1.3, respectively, to clearly visualize the pattern evolution at lower laser intensities.